The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Radio Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 Hz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes.
LTE uses Orthogonal Frequency Division Multiplexing, OFDM, in the downlink and Discrete Fourier Transform, DFT-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI) indicated by the physical CFI channel (PCFICH) transmitted in the first symbol of the control region. The control region also contains physical downlink control channels (PDCCH) and possibly also physical Hybrid Automatic Repeat Request (HARQ) indication channels (PHICH) carrying ACK/NACK for the uplink transmission.
The downlink subframe also contains common reference symbols (CRS), also referred to as cell-specific reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
In LTE, two radio frame structures are supported: Type 1 applicable to FDD (Frequency Division Duplex), and type 2 applicable to TDD (Time Division Duplex). A type 2 frame structure is illustrated in FIG. 6. In both frame structure types, each radio frame of 10 ms is divided into two half-frames of 5 ms, and each half-frame consists of five subframes of length 1 ms. In frame structure type 2, each subframe is either a downlink subframe, an uplink subframe or a special subframe giving rise to different TDD configurations, as shown e.g. in FIG. 7. The special subframe provides a guard period when switching from downlink to uplink transmission, or vice versa.
The supported uplink-downlink configurations in LTE TDD are listed in Table 1 where, for each subframe in a radio frame, “D” denotes the subframe is reserved for downlink transmissions, “U” denotes the subframe is reserved for uplink transmissions and “S” denotes a special subframe with the three fields: A downlink part, DwPTS, a guard period, GP and an uplink part UpPTS, which are shown in FIG. 6. The length of DwPTS and UpPTS is given by Table 1 subject to the total length of DwPTS, GP and UpPTS being equal to 1 ms. Each subframe consists of two slots, each of length 0.5 ms.
Uplink-downlink configurations with both 5 ms and 10 ms downlink-to-uplink switch-point periodicity are supported. In case of 5 ms downlink-to-uplink switch-point periodicity, the special subframe exists in both half-frames.
In case of 10 ms downlink-to-uplink switch-point periodicity, the special subframe exists in the first half-frame only. Subframes 0 and 5 and DwPTS are always reserved for downlink transmission. UpPTS and the subframe immediately following the special subframe are always reserved for uplink transmission.
In a TDD cell, a TDD configuration is characterized by both uplink-downlink configuration and special subframe configuration. Therefore, the term TDD configuration used hereinafter refers to a combination of uplink-downlink configuration and special subframe configuration. It should be noted that more TDD configurations than the ones listed in table 1 may be introduced in the future. The herein described embodiments are not limited to the existing TDD configurations, rather they are equally applicable to new configurations that may be defined in future.
TABLE 1Uplink-downlink configurationsDownlink-to-UplinkSwitch-pointSubframe numberperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
Dynamic TDD and (F)eICIC
Dynamic TDD is currently discussed in 3GPP. With dynamic TDD, each cell can switch its UL-DL configuration according to instantaneous traffic demand. In dynamic TDD, the subframes can be divided into two types: static subframes and flexible subframes. The static subframes have fixed link directions, i.e. they are either designated as static downlink subframes, or static uplink subframes, while flexible subframes can be dynamically changed between the uplink (UL) and downlink (DL) directions.
Considering user equipment (UE) reception (DL) in the two types of subframes, the interference situations may be different. In static DL subframes, the inter-cell interference comes from the neighboring eNB(s) while in flexible subframes the inter-cell interference could either come from neighboring eNB(s) or certain UE(s) served by the neighboring eNB(s) who are currently scheduled with UL transmissions. To capture the different interference situations, separate CSI measurements are preferred for the two types of subframes so that DL scheduling as well as link adaptation can be properly performed.
Enhanced inter-cell interference coordination (eICIC) and its further evolvement (Further enhanced ICIC, or FeICIC) aims to improve the UE performance in cell range expansion (CRE) areas within a heterogeneous network. A UE in the CRE area will experience severe interference from the high power macro base station. With the introduction of time domain ICIC, e.g. almost blank subframe (ABS), the transmission from the macro eNB could be eliminated to a large extent in a certain subset of subframes. Hence, the interference to a pico user is reduced. Similar to dynamic TDD, separate CSI measurement in ABS subframes and non-ABS subframes are specified.
CSI Measurement
For a UE in transmission modes 1-8 or in transmission mode 9 when the parameter pmi-RI-Report is not configured by higher layers, the UE shall derive the channel measurements for computing Channel Quality Information (CQI) based on CRS. For a UE in transmission mode 9 when parameter pmi-RI-Report is configured by higher layers, the UE shall derive the channel measurements based on non-zero power Channel-State Information (CSI) reference signals (NZP CSI-RS). For the above cases (transmission modes 1-9), it is not specified in current standard specifications how to perform interference measurements, but according to common understanding, the interference measurement is based on CRS.
In addition to “conventional” non-zero-power CSI-RS, there is also the possibility to configure a terminal in transmission mode 1-9 with one set of zero-power CSI-RS resources, which has the same structure as non-zero-power CSI-RS resources. Zero-power as well as non-zero-power CSI-RS resource configurations may be associated with:                A certain periodicity (e.g. 5 ms, 10 ms, 20 ms, 40 ms, or 80 ms); and        A certain subframe offset within the period; and        A certain configuration within a resource block pair,The zero-power CSI-RS resources may, for example, correspond to (non-zero power) CSI-RS of other terminals within the cell, or within neighboring cells. Thus, despite the name, the zero-power CSI-RS resources do not necessarily have zero power. The zero-power CSI-RS resources may also correspond to CSI-IM resources, which will be described in more detail below.        
UEs configured in transmission mode 10 may be configured with one or more zero-power CSI-RS resource configuration(s).
For a UE in transmission mode 10, one or more CSI processes can be configured for a UE per serving cell. A CSI reported by the UE corresponds to a CSI process. CSI processes are configured via Radio Resource Control (RRC) signaling, and may be configured with or without PMI/RI reporting. By utilizing two or more CSI processes, the network node may for example configure the UE to provide CSI reports corresponding to different transmission hypotheses, or to different interference conditions.
According to the current specification (3GPP TS 36.213 version 11.3.0), each CSI process is associated with a CSI-RS resource (defined in Section 7.2.5 of TS 36.213). For each CSI process, the UE shall derive the channel measurements based on only the non-zero power CSI-RS within a configured CSI-RS resource associated with the CSI process.
Each CSI process is further associated with a CSI-interference measurement (CSI-IM) resource (defined in Section 7.2.6 of TS 36.213). A CSI-IM configuration is associated with a zero-power CSI RS configuration, and the UE shall derive the interference measurements based on only the zero power CSI-RS within the configured CSI-IM resource associated with the CSI process.
Note that in future versions of the standard, it is not excluded that the UE could derive channel measurements and/or interference measurements based on additional parameters as well.
Two CSI subframe sets may also be configured for a CSI process, enabling the UE to do separate CSI measurements in the different subsets of subframes. The CSI subframe sets are defined for each CSI process in Transmission Mode (TM) 10. It is not excluded that more than two subframe sets will be available in future versions of the standard.